The present invention relates to spectral energy sources, and more particularly, to a tunable spectral energy source that uses the dispersive effect of electromagnetic energy to select spectral components that are emitted from the spectral source.
One type of device for isolating single spectral bands of broad band illumination sources is a monochromator. These devices isolate and emit a narrow region of a spectrum. Spectral tuning is accomplished by rotating the diffraction grating to the angle corresponding to the desired emission wavelength. Spectra can be constructed by sequentially tuning the diffraction grating through the necessary wavelength range. The intensity at each wavelength is controlled by the length of time the grating remains at the corresponding angle. Practically, the sequential nature of this procedure limits the usefulness of monochromators for simulating spectral sources because the time required to construct the spectrum may often exceed the sensor integration times.
Another type of system for isolating a particular wavelength of light in the infrared, visible, or ultraviolet regions is based upon a technique known as xe2x80x9caperture filling.xe2x80x9d Basic physics principals of aperture filling state that binary amplitude gratings can be used to expand the light from an aperture to uniformly fill an aperture with a width less than or equal to 4 times the width of the small aperture. Conversely, light from a large aperture can be concentrated into a sub-aperture with a width greater than or equal to 0.25 times the width of the large aperture. This technique to concentrate red, green and blue light from a large aperture into three sub-apertures. The intensity of the light in each sub-aperture is controlled with digital micro mirror or liquid crystal devices. Aperture filling limits the number of sub-apertures to four, which in turn limits the technique to applications requiring four or less spectral bands. Because this technique is constrained to a corresponding fill factor or 0.25 or greater, only four controllable wavelength bands may be emitted at a single time.
At present there is no device that can be operated quickly and electronically to simulate random complex spectra covering a wide wavelength band. Therefore, a need exists for a tunable spectral source that can emit spectra over a wide bandwidth, particularly, optical spectra ranging from long infrared to ultraviolet light.
The present invention provides a tunable spectral source for emitting optical energy characterized by a particular bandwidth, or spectral range. The invention takes advantage of the fact that broadband light such as white light may be refracted, or broken down into it multi-spectral, or multi-colored spectral components. The tunable spectral source includes an enclosure having first and second apertures; an optical dispersive element positioned in the enclosure; and multiple pixel source elements that are individually controllable for selectively directing one or more broadband light signals through the first aperture to irradiate the optical dispersive element. Each of the broadband light signals irradiates the optical dispersive element at a unique angle of incidence. The optical dispersive element disperses the broadband light signals into spectral component signals at dispersion angles that are dependent upon the angle of incidence of each broadband light signal that irradiates the optical dispersive element. The portions of the spectral component signals that are emitted through the second aperture are determined by selecting one or more particular broadband pixel source elements to irradiate the optical dispersive element.